Normal form bisimulation, also known as open bisimulation, is a coinductive technique for higher-order program equivalence in which programs are compared by looking at their essentially infinitary tree-like normal forms, i.e. at their Bohm or Levy-Longo trees. The technique has been shown to be useful not only when proving metatheorems about \(\lambda \)-calculi and their semantics, but also when looking at concrete examples of terms. In this paper, we show that there is a way to generalise normal form bisimulation to calculi with algebraic effects, a la Plotkin and Power. We show that some mild conditions on monads and relators, which have already been shown to guarantee effectful applicative bisimilarity to be a congruence relation, are enough to prove that the obtained notion of bisimilarity, which we call effectful normal form bisimilarity, is a congruence relation, and thus sound for contextual equivalence. Additionally, contrary to applicative bisimilarity, normal form bisimilarity allows for enhancements of the bisimulation proof method, hence proving a powerful reasoning principle for effectful programming languages.

/pdf/effectful-normal-form-bisimulation-7q76a07gfk.pdf

Effectful Normal Form Bisimulation

This paper studies quantitative refinements of Abramsky's applicative similarity and bisimilarity in the context of a generalisation of Fuzz, a call-by-value λ-calculus with a linear type system that can express program sensitivity, enriched with algebraic operations a la Plotkin and Power. To do so a general, abstract framework for studying behavioural relations taking values over quantales is introduced according to Lawvere's analysis of generalised metric spaces. Barr's notion of relator (or lax extension) is then extended to quantale-valued relations, adapting and extending results from the field of monoidal topology. Abstract notions of quantale-valued effectful applicative similarity and bisimilarity are then defined and proved to be a compatible generalised metric (in the sense of Lawvere) and pseudometric, respectively, under mild conditions.

/pdf/quantitative-behavioural-reasoning-for-higher-order-58e7occag7.pdf

Quantitative Behavioural Reasoning for Higher-order Effectful Programs: Applicative Distances

The paper investigates behavioural equivalence between programs in a call-by-value functional language extended with a signature of (algebraic) effect-triggering operations. Two programs are considered as being behaviourally equivalent if they enjoy the same behavioural properties. To formulate this, we define a logic whose formulas specify behavioural properties. A crucial ingredient is a collection of modalities expressing effect-specific aspects of behaviour. We give a general theory of such modalities. If two conditions, openness and decomposability, are satisfied by the modalities then the logically specified behavioural equivalence coincides with a modality-defined notion of applicative bisimilarity, which can be proven to be a congruence by a generalisation of Howe’s method. We show that the openness and decomposability conditions hold for several examples of algebraic effects: nondeterminism, probabilistic choice, global store and input/output.

/pdf/behavioural-equivalence-via-modalities-for-algebraic-effects-3wzj3qovuj.pdf

Behavioural equivalence via modalities for algebraic effects

Weakest Preconditions in Fibrations.

This dissertation investigates notions of program equivalence and metric for higher-order sequential languages with algebraic effects.
Computational effects are those aspects of computation that involve forms of interaction with the environment. Due to such an interactive behaviour, reasoning about effectful programs is well-known to be hard. This is especially true for higher-order effectful languages, where programs can be passed as input to, and returned as output by other programs, as well as perform side-effects. Additionally, when dealing with effectful languages, program equivalence is oftentimes too coarse, not allowing, for instance, to quantify the observable differences between programs. A natural way to overcome this problem is to re ne the notion of a program equivalence into the one of a program distance or program metric, this way allowing for a finer, quantitative analysis of program behaviour. A proper account of program distance, however, requires a more sophisticated theory than program equivalence, both conceptually and mathematically. This often makes the study of program distance way more di cult than the corresponding study of program equivalence.

Coinductive Equivalences and Metrics for Higher-order Languages with Algebraic Effects

The article investigates behavioural equivalence between programs in a call-by-value functional language extended with a signature of (algebraic) effect-triggering operations. Two programs are considered as being behaviourally equivalent if they enjoy the same behavioural properties. To formulate this, we define a logic whose formulas specify behavioural properties. A crucial ingredient is a collection of modalities expressing effect-specific aspects of behaviour. We give a general theory of such modalities. If two conditions, openness and decomposability, are satisfied by the modalities, then the logically specified behavioural equivalence coincides with a modality-defined notion of applicative bisimilarity, which can be proven to be a congruence by a generalisation of Howe’s method. We show that the openness and decomposability conditions hold for several examples of algebraic effects: nondeterminism, probabilistic choice, global store, and input/output.

https://dl.acm.org/doi/pdf/10.1145/3363518

Behavioural Equivalence via Modalities for Algebraic Effects

Quantitative Logics for Equivalence of Effectful Programs

In order to reason about effects, we can define quantitative formulas to describe behavioural aspects of effectful programs. These formulas can for example express probabilities that (or sets of correct starting states for which) a program satisfies a property. Fundamental to this approach is the notion of quantitative modality, which is used to lift a property on values to a property on computations. Taking all formulas together, we say that two terms are equivalent if they satisfy all formulas to the same quantitative degree. Under sufficient conditions on the quantitative modalities, this equivalence is equal to a notion of Abramsky's applicative bisimilarity, and is moreover a congruence. We investigate these results in the context of Levy's call-by-push-value with general recursion and algebraic effects. In particular, the results apply to (combinations of) nondeterministic choice, probabilistic choice, and global store.

Niels Voorneveld

Papers

Behavioural equivalence via modalities for algebraic effects

Behavioural Equivalence via Modalities for Algebraic Effects

Quantitative Logics for Equivalence of Effectful Programs

Quantitative Logics for Equivalence of Effectful Programs

Extensions of Scott’s Graph Model and Kleene’s Second Algebra